The present invention relates to the combustion of hydrocarbonaceous fuel such as coal in, for instance, a furnace for generation of power, and to the reduction of generation of oxides of nitrogen in the course of that combustion.
Combustion of coal in the furnaces of power plants continues to be a significant means of generating energy. As that combustion continues to be believed to cause atmospheric emissions of NOx which continue to be considered to contribute to atmospheric pollution, there is still substantial interest in identifying ways to reduce the amount of NOx emitted to the atmosphere in the course of that combustion.
One known method to reduce NOx emissions from boilers and furnaces is to inject a reagent such as ammonia, urea, cyanuric acid or ammonium carbonate into the combustion chamber, whereupon the reagent forms amine radicals (xe2x80x94NH2) at high temperature and reacts with NO present in the high temperature combustion gases in the combustion chamber to form N2. This method is well known as the selective non-catalytic reduction (SNCR) process and is described in numerous aspects in the prior art. Prominent among SNCR processes are those described by Lyon in U.S. Pat. No. 3,900,554 and by Arand et. al. in U.S. Pat. Nos. 4,208,386 and 4,325,924. Recent improvements in the SNCR process include those described in U.S. Pat. No. 6,030,204 and U.S. patent application Publication No. US 2002/0025285 A1. Ammonia and urea are the preferred reagents. For effective reduction of NOx, the reagent has to be mixed uniformly with the combustion gases containing NOx within the space and residence time available for each combustion process. Uniform mixing is a difficult practical problem as the molar ratio of the reagent to flue gas is on the order of 1,000 to 10,000 for flue gas containing 100 to 1000 ppm of NOx.
The efficiency of NOx reduction by the SNCR process is strongly dependent on the temperature at which the reaction is carried out. The effective reaction temperature is conventionally believed to be about 1600 to 2000 F.; carried out at this temperature, the SNCR reaction can provide NOx reduction of up to 90% in a small well mixed system. However, in large boilers the SNCR reaction provides maximum NOx reduction of about 55 to 60% due to the large temperature gradient within a boiler and also to the physical limitations to achieving uniform mixing and to providing adequate mixing time. The ideal gas temperature at which to inject the-reducing reagent into a boiler is typically believed to be between 1800-2000 F., considering the rapid cooling that the flue gas undergoes as it approaches the convective section of the boiler. The amount of NOx reduction achieved drops sharply at temperatures below or above the 1800-2000 F. range. Emissions of unreacted reagent, conventionally termed xe2x80x9cammonia slipxe2x80x9d, is a problem at lower temperatures and the sharp drop of NOx reduction efficiency is the problem at higher temperature.
Unfortunately, in conventional furnaces such as utility boilers, the temperature of the gases (composed chiefly of gaseous combustion products and nitrogen) as they exit the combustion zone of the furnace (i.e. the zone where the combustion of the fuel takes place) and enter the reduction zone (where the injection and reaction of the SNCR reagent occurs) is typically about 2100 to 2300 F. As a result, NOx reduction of only 15 to 30% is typically achieved even when the reagent is mixed well with the flue gas.
Up to now, attempting to remedy this situation by lowering the temperature of the gaseous combustion products leaving the combustion zone risks sacrificing the efficiency or output of the furnace, without cumbersome and expensive modifications to the entire system. Although the amount of NOx reduction can be improved to a certain degree by increasing the amount of ammonia or other reagent that is fed to the reduction zone, this is an expensive solution. On the other hand, adding conventional means such as low-NOx burners and over fire air ports for lowering NOx formation (thereby lessening the amount of reagent that needs to be injected) are generally understood to delay the combustion process and increase the temperature of the combustion products leaving the combustion zone. Thus, the overall efficiency of the SNCR process suffers even with the lessened reagent requirements.
In the SNCR process, reagent is injected from an array of nozzles to achieve good mixing with the hot furnace gas located in the optimum temperature range available for the boiler or the furnace. For boiler applications, the zone at the end of the radiant section is the preferred location. Gas temperature is dropping rapidly as it flows from the radiant furnace section to the convective bank; however, injection of the reagent within the convective bank is impractical because of the heat exchanger tubes found there. The location and distribution of the reagent injection nozzles are the most important parameter as the reagent, typically supplied as an aqueous solution, must be atomized, vaporized, mixed with flue gas, and reacted with NO within the relatively short residence time available in the tail end of the boiler furnace. Injection velocity, directions, and droplet size distributions all influence the efficiency of NOx reduction and need to be optimized for each boiler. Since the gas temperature changes with the boiler load, reagent nozzles are often located in multiple levels and the injection level is controlled as the optimum temperature range shifts.
Flue gas recirculation (FGR) in the upper furnace zone is an effective way to cool down the flue gas and is sometimes used to control the steam temperature at the superheater. In this method flue gas after the air heater is recirculated by an induced draft fan. This technique, however, presents the technical issue of the wear to and maintenance of the FGR fan caused by the ash in the flue gas. The economics is another issue, because of the expense of installing the large fan that would be necessary to handle warm flue gas (about 400 to 500 F.) and the expense of the power required for recirculation.
It is also possible to inject a diluent to cool down the gaseous combustion products leaving the combustion zone. A water spray could be used as an effective coolant. However, any addition of diluents, other than recirculated flue gas, would result in a significant energy penalty and reduce the thermal efficiency of the boiler.
Thus, there remains a need for methods that provide the benefits of the SNCR process while achieving, and not sacrificing, other benefits.
The present invention satisfies this need and affords the advantages described herein. According to the invention, hydrocarbonaceous fuel is combusted in a furnace having a combustion zone, burner means for combusting hydrocarbonaceous fuel in said combustion zone to generate heat of combustion and gaseous combustion products containing NOx, feed means for feeding said fuel and combustion air to said burner means, and a reduction zone downstream from said combustion zone into which said gaseous combustion products pass from said combustion zone, wherein combustion products at a temperature above 1900 F. arepresent in said reduction zone, and a reducing reagent is injected into said reduction zone and reacts there with NOx in said combustion products to form N2 and thereby lessen the amount of NOx that would otherwise be emitted from said furnace. Oxygen is fed into said fuel, by injecting it directly into said fuel as said fuel emerges from said burner or by adding it to the air that is fed through said burner, while reducing the amount of combustion air fed through said burner by an amount containing sufficient oxygen that the overall combustion zone stoichiometric ratio varies by not more than 10% compared to the stoichiometric ratio without said addition of oxygen, and combusting the fuel in said air and said oxygen,
wherein the amount of said oxygen is sufficient to lower the temperature of said combustion products passing into said reduction zone but not to a temperature below 1800 F., and the amount of said oxygen is less than 25% of the stoichiometric amount required for complete combustion of said fuel.
Preferably, air is added from a source other than said burner into a region within said combustion chamber outside said fuel-rich zone, in an amount containing at least sufficient oxygen that the total amount of oxygen fed into said combustion chamber is at least the stoichiometric amount needed for complete combustion of said fuel.
While the combustion products in some region(s) of the reduction may be below 1900 F., some are above that temperature. The injection of oxygen as described herein brings about a reduction of the temperature of the combustion products that pass into the reduction zone, while also providing the other advantages described herein.
In some preferred embodiments, particularly those wherein said fuel contains bound nitrogen, said combustion is staged with a low NOx burner and the fuel rich flame zone stoichiometric ratio is between 0.6 and 1.0, or said combustion is staged with over fire air and the primary combustion zone stoichiometric ratio is between 0.6 and 1.0.
In other preferred embodiments, a stream of fuel is fed through said burner and oxygen is fed into said fuel by injecting it through a lance positioned in said stream, into the fuel as the fuel emerges from the burner. In yet other preferred embodiments, a stream of fuel is fed through an annular fuel passage of said burner, and oxygen is fed into said fuel by injecting it through an annular passage surrounding or surrounded by said annular fuel passage.
In the most preferred embodiments, the fuel is coal.
The invention provides numerous advantages.
Emission of NOx per unit of energy generated is reduced. Consumption of the SNCR reagent is reduced for the same amount of NOx reduction. The thermal efficiency of the furnace improves: more heat is transferred, thereby lowering the flue gas temperature. Also, more energy is recovered from the same input of fuel, which also enables fuel input to be reduced slightly while maintaining the same overall heat transfer to the boiler tubes. The combined process reduces the NOx emissions through the unexpectedly synergistic combination of two processes.
As used herein, xe2x80x9cstoichiometric ratioxe2x80x9d means the ratio of oxygen fed, to the total amount of oxygen that would be necessary to convert fully all carbon, sulfur and hydrogen present in the substances comprising the feed to carbon dioxide and sulfur dioxide, and water.
As used herein, xe2x80x9cNOxxe2x80x9d means oxides of nitrogen such as but not limited to NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof.
As used herein, xe2x80x9cbound nitrogenxe2x80x9d means nitrogen that is part of a molecule that also contains carbon and hydrogen and optionally also oxygen.
As used herein, xe2x80x9creducing reagentxe2x80x9d means any nitrogenous compound that reacts with NO at 1900xc2x0 F. to form reaction products that contain N2.
As used herein, xe2x80x9cstaged combustion with low NOx burnersxe2x80x9d means combustion in a furnace wherein mixing with fuel of a portion of the combustion air required for complete combustion of the fuel is delayed to produce a flame with a relatively large fuel rich flame zone.
As used herein, xe2x80x9cglobally staged combustion or staged combustion with over fire airxe2x80x9d means combustion in a furnace wherein a portion of the combustion air (the xe2x80x9cover fire airxe2x80x9d) required for complete combustion of the fuel is fed to the furnace not through or immediately adjacent any burner but instead through one or more inlets situated between the burner(s) and the furnace flue means, and is fed without an associated feed of fuel.